The hydrophobicity and inert characteristics of polyolefin (PO) surfaces cause many problems in the assembly of PO-based substrates with dissimilar materials, especially polar materials. Hot-melt adhesives are often used because such adhesives are able to penetrate into the PO surface to create a good fusion bond with the PO matrix. Grafted PO (g-PO), the most popular coupling agent for PO composites and their blends, has been used in the formulation of hot-melt adhesives for joining PO-based substrates with dissimilar materials since grafted-PO contains polar groups that benefit adhesion with polar materials. To promote the fusion bonding process, hot-gun, hotplate, ultrasonic, vibration, and resistance welding may be applied.
For PO composites, especially in structural applications, adhesion at a structural joint must be strong enough to carry a load transferred into/through the joint. Current commercial hot-melt adhesives cannot sustain high load levels. Furthermore, the composition of conventional hot-melt adhesives is controlled to impart low melting point for easy operation, high flowability for good wetting with the substrates, and high tackicity for promoting adhesion. Therefore, adhesive formulas for conventional hot-melt adhesives generally comprise a large portion of low molecular weight polymer with/without functional groups and/or a polar wax. Thus, the joint is very sensitive to temperature, moisture, and other environmental effects, especially in long-term service.
Japanese Patent Abstract 58052340 published Mar. 28, 1983 describes an adhesive polyolefin resin composition comprising a graft-modified olefin polymer, an unmodified olefin polymer and 0.1-10 parts by weight magnesium oxide.
For polyolefin-based composite structures, the required bond strength is often much greater than with conventional plastics since composite structures are often used in high performance applications. Thus, there remains a need for an effective hot-melt adhesive that provides good bond strength under a variety of environmental conditions, especially for bonds between a polyolefin-based substrate and materials dissimilar to the polyolefin-based substrate.